Platinum(II) diimine complexes with halide/pseudohalide ligands and dangling trialkylamine or ammonium groups

Article abstract of DOI:10.1021/ic202462a

A series of platinum(II) complexes with the formulas Pt(diimine)(pip ₂NCNH₂)(L)²⁺ [pip₂NCNH ₂⁺ = 2,6-bis(piperidiniummethyl)phenyl cation; L = Cl, Br, I, NCS, OCN, and NO₂; diimine = 1,10-phenanthroline (phen), 5-nitro-1,10-phenanthroline (NO₂phen), and 5,5′- ditrifluoromethyl-2,2′-bipyridine (dtfmbpy)] were prepared by the treatment of Pt(pip₂NCN)Cl with a silver(I) salt followed by the addition of the diimine and halide/pseudohalide under acidic conditions. Crystallographic data as well as ¹H NMR spectra establish that the metal center is bonded to a bidentate phenanthroline and a monodentate halide/pseudohalide. The pip₂NCNH₂⁺ ligand with protonated piperidyl groups is monodentate and bonded to the platinum through the phenyl ring. Structural and spectroscopic data indicate that the halide/pseudohalide group (L^-) and the metal center in Pt(phen)(pip₂NCNH₂)(L)²⁺ behave as Bronsted bases, forming intramolecular NH...L/NH...Pt interactions involving the piperidinium groups. A close examination of the 10 structures reported here reveals linear correlations between N-H...Pt/L angles and H...Pt/L distances. In most cases, the N-H bond is directed toward the Pt-L bond, thereby giving the appearance that the proton bridges the Pt and L groups. In contrast to observations for Pt(tpy)(pip₂NCN) ⁺ (tpy = 2,2′;6′,2″-terpyridine), the electrochemical oxidation of deprotonated adducts, Pt(diimine)(L)(pip ₂NCN), is chemically and electrochemically irreversible.

Full text of DOI:10.1021/ic202462a

Article
pubs.acs.org/IC
Platinum(II) Diimine Complexes with Halide/Pseudohalide Ligands
and Dangling Trialkylamine or Ammonium Groups
Sayandev Chatterjee, Jeanette A. Krause, Kumudu Madduma-Liyanage, and William B. Connick*
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States
S
* Supporting Information
ABSTRACT: A series of platinum(II) complexes with the
formulas Pt(diimine)(pip2NCNH2)(L)2+ [pip2NCNH2+ = 2,6-
bis(piperidiniummethyl)phenyl cation; L = Cl, Br, I, NCS,
OCN, and NO2; diimine = 1,10-phenanthroline (phen), 5-
nitro-1,10-phenanthroline (NO2phen), and 5,5′-ditrifluoro-
methyl-2,2′-bipyridine (dtfmbpy)] were prepared by the
treatment of Pt(pip2NCN)Cl with a silver(I) salt followed
by the addition of the diimine and halide/pseudohalide under
1
acidic conditions. Crystallographic data as well as H NMR
spectra establish that the metal center is bonded to a bidentate
phenanthroline and a monodentate halide/pseudohalide. The
+
pip2NCNH2 ligand with protonated piperidyl groups is monodentate and bonded to the platinum through the phenyl ring.
Structural and spectroscopic data indicate that the halide/pseudohalide group (L−) and the metal center in Pt(phen)-
(pip2NCNH2)(L)2+ behave as Brønsted bases, forming intramolecular NH···L/NH···Pt interactions involving the piperidinium
groups. A close examination of the 10 structures reported here reveals linear correlations between N−H···Pt/L angles and
H···Pt/L distances. In most cases, the N−H bond is directed toward the Pt−L bond, thereby giving the appearance that the
proton bridges the Pt and L groups. In contrast to observations for Pt(tpy)(pip2NCN)+ (tpy = 2,2′;6′,2″-terpyridine), the
electrochemical oxidation of deprotonated adducts, Pt(diimine)(L)(pip2NCN), is chemically and electrochemically irreversible.
The remarkable near-reversibility of the electrochemistry of
Pt(tpy)(pip2NCN)+ exceeds expectations based on the large
geometry change.26 This observation, as well as spectroscopic
and crystallographic data, has led us to suggest that the dangling
piperidyl groups interact with the PtII center prior to or during
electron transfer, resulting in a preorganized molecule that
undergoes more rapid electron transfer because of its lower
Marcus reorganization energy.28 To better understand the
interactions of the metal center with dangling nucleophiles and
electrophiles, we have sought to prepare structural and
electronic analogues of Pt(tpy)(pip2NCN)+, such as Pt(phen)-
(pipNC)2 (Scheme 1).29 The preparation and handling of such
compounds presents challenges because of difficulties in forcing
the metal to preferentially bond to one nucleophile over
another. We have recently demonstrated that protonation of
the dangling nucleophilic groups is an effective strategy for
preventing coordination to the metal center.29 Herein we
describe the synthesis and properties of the diprotonated
Pt(diimine)(pip2NCNH2)(L)2+ complexes (Scheme 1), where
L− is a halide or pseudohalide ligand and the diimine ligand is a
1,10-phenanthrolinyl, 5-nitro-1,10-phenanthrolinyl, or 5,5′-
ditrifluoromethyl-2,2′-bipyridyl group. The deprotonated ad-
ducts, Pt(diimine)(pip2NCN)(L), can be regarded as close
analogues of Pt(tpy)(pip2NCN)+, in which the terpyridyl ligand
INTRODUCTION
■
Square-planar d8-electron and octahedral d6-electron second-
and third-row transition-metal complexes are promising
multiredox catalysts because of their tendency to undergo
cooperative two-electron changes in the oxidation state coupled
to bond-making and bond-breaking steps.1,2 Fine control of the
d6/d8 redox potentials would seem a logical strategy for
improving catalysts and rationally designing new catalysts.
However, surprisingly little is known about the outer-sphere
electron-transfer reactions associated with this couple, in large
part because these reactions are typically irreversible because of
the accompanying drastic changes in the metal coordination
sphere. Building on related studies,3−25 we have prepared
Pt(tpy)(pip2NCN)+ (Scheme 1), which is the first example of a
nearly cooperative outer-sphere two-electron platinum re-
agent.26,27 In the d8-electron configuration, the complex
possesses two dangling nucleophilic piperidyl groups that can
interact with the square-planar metal center and help to
stabilize the octahedral coordination geometry preferred by the
d6-electron configuration. The complex undergoes a nearly
chemically and electrochemically reversible two-electron
oxidation at 0.4 V vs Ag/AgCl (0.1 M TBAPF6 in acetonitrile).
The separation between the anodic and cathodic peaks of the
oxidation wave (ΔEp) decreases with the scan rate (150−43
mV; 20.5−0.01 V/s), as expected for an accompanying
structural reorganization involving the formation of two PtIV−
piperidyl bonds.
Received: November 15, 2011
Published: April 2, 2012
© 2012 American Chemical Society
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Article
Scheme 1. Pt(tpy)(pip2NCN)+, Pt(phen)(pipNC)2, and
[Pt(diimine)(pip2NCNH2)(L)](PF6)2 (1−8)
finger dewar. Emission spectra were corrected for instrumental
response. Mass spectra were obtained by electrospray ionization
(ESI) of acetonitrile solutions using a Micromass Q-TOF-2 instru-
ment.
Cyclic voltammetry measurements were carried out using a standard
three-electrode cell and a 100 B/W electrochemical workstation from
Bioanalytical Systems. Scans were recorded on CH2Cl2 solutions
containing 0.1 M TBAPF6. All scans were recorded using a platinum
wire auxiliary electrode and a 0.79 mm2 gold working electrode. Scans
recorded using platinum and glassy carbon working electrodes were
significantly broadened, suggesting poorer electrochemical reversi-
bility. Between measurements, the working electrode was polished
with 0.05 μm alumina, rinsed with distilled water, and wiped dry using
a Kimwipe. Reported potentials are referenced vs Ag/AgCl (3.0 M
NaCl). Peak currents (ip) were estimated with respect to the
extrapolated baseline current, as described by Kissinger and Heine-
man.37 Under these conditions, the ferrocene/ferrocenium (Fc/Fc+)
couple occurs at 0.51 V (ΔEp, 68 mV; 0.10 V/s).
[Pt(phen)(pip2NCNH2)Cl](PF6)2 (1). AgPF6 (25 mg, ∼0.1 mmol)
was added to a solution of Pt(pip2NCN)Cl (50 mg, 0.1 mmol) in
CH3CN, and the mixture was stirred in the dark for 1 h. The resulting
precipitate was removed by filtration through Celite. Trifluoroacetic
acid (14.7 μL, 0.2 mmol) was added to the pale-yellow filtrate,
followed by the addition of 1,10-phenanthroline (phen; 18 mg, 0.1
mmol) and KCl (8 mg, 0.1 mmol). The mixture was refluxed for 2 h
and then filtered. Excess NH4PF6 was added to the filtrate, and the
mixture was filtered again to remove residual NH4PF6. The filtrate was
rotary evaporated to one-third of the initial volume, excess diethyl
ether was added, and the solution was cooled in a refrigerator. The
resulting pale-yellow precipitate was collected and washed with water
to give a pale-yellow powder. Yield: 0.066 g, 68%. 1H NMR (CD3CN,
δ): 1.42 (2H, m, aliphatic pip2NCNH2), 1.62 (2H, m, aliphatic
pip2NCNH2), 1.77 (4H, m, aliphatic pip2NCNH2), 1.87 (4H, m,
aliphatic pip2NCNH2), 2.84 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H
has been replaced by L− and a diimine ligand. The results
presented here suggest that the metal center in this series of
complexes is capable of acting as a Brønsted base and that these
interactions have a profound influence on the structural,
electronic, and electron-transfer properties.
= 14.8 and 12.0 Hz), 3.03 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H
15.2 and 11.6 Hz), 3.29 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H
=
=
EXPERIMENTAL SECTION
11.6 and 2 Hz), 3.64 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 10.8
and 4 Hz), 4.17 (2H, dd, benzylic CH2, JH−H = 12.8 and 2.0 Hz), 4.95
(2H, dd, benzylic CH2, JH−H = 12.8 and 2.0 Hz), 7.29 (1H, t, aromatic
pip2NCNH2, JH−H = 8.0 Hz), 7.41 (2H, d, aromatic pip2NCNH2, JH−H
= 8.0 Hz), 7.66 (1H, d, phen, JH−H = 6.8 Hz), 7.67 (1H, d, phen, JH−H
■
General Considerations. K2PtCl4 was obtained from Pressure
Chemical Company (Pittsburgh, PA). 2,2′-Azobis(isobutyronitrile)
(AIBN), 2-bromo-m-xylene, and 1,5-cyclooctadiene (COD) were
obtained from Aldrich Chemical Co. (Milwaukee, WI). All other
reagents were purchased from Acros. Tetrahydrofuran (THF) was
distilled from sodium metal and benzophenone, and ethanol was
distilled from zinc metal and potassium hydroxide. All other chemicals
were used as received. Pt(COD)Cl2,30 Pt(DMSO)2Cl2,31 2,6-
bis(bromomethyl)bromobenzene,32 2,6-[CH2(C5H10N)]2C6H3Br
(pip2NCNBr),33 2-[CH2(C5H10N)]C6H4 (pipNCH),29 5,5′-di-
(trifluoromethyl)-2,2′-bipyridine (dtfmbpy),34 and Pt(pip2NCN)Cl35
and Pt(pipNC)(DMSO)Cl36 [pip2NCN− = 2,6-bis(piperidylmethyl)-
phenyl anion; pipNC− = 2-(piperidylmethyl)phenyl anion] were
prepared according to literature procedures. Syntheses involving
amines were carried out in an inert argon atmosphere using standard
Schlenk techniques. Argon was pre-dried using activated sieves, and
trace impurities of oxygen were removed with activated R3-11 catalyst
from Schweizerhall.
= 6.8 Hz), 7.94 (1H, dd with Pt satellites, phen, JH−Pt = 61 Hz, JH−H
=
6.8 and 1.2 Hz), 8.20 (1H, t, phen, JH−H = 6.8 Hz), 8.21 (2H, s, N−
H), 8.22 (1H, t, phen, JH−H = 6.8 Hz), 8.82 (1H, dd, phen, JH−H = 8.4
and 0.8 Hz), 8.92 (1H, dd, phen, JH−H = 8.4 and 0.8 Hz), 9.48 (1H, dd,
1
phen, JH−H = 6.8 and 0.8 Hz). H NMR (CD2Cl2, δ): 0.72−1.52 (m,
aliphatic pip2NCNH2 overlapping with solvent resonances), 1.52−1.98
(8H, m, aliphatic pip2NCNH2 β-H), 2.81 (2H, dd, aliphatic
pip2NCNH2 α-H, JH−H = 20.0 and 7.6 Hz), 3.05 (2H, dd, aliphatic
pip2NCNH2 α-H, JH−H = 24.0 and 8.0 Hz), 3.33 (2H, dd, aliphatic
pip2NCNH2 α-H, JH−H = 24.0 and 8.0 Hz), 3.69 (2H, dd, aliphatic
pip2NCNH2 α-H, JH−H = 11.2 and 3.6 Hz), 4.10 (2H, dd, benzylic
CH2, JH−H = 14.0 and 9.6 Hz), 5.15 (2H, dd, benzylic CH2, JH−H
=
13.2 and 1.2 Hz), 7.28 (1H, t, aromatic pip2NCNH2, JH−H = 8.0 Hz),
7.37 (2H, d, aromatic pip2NCNH2, JH−H = 8.0 Hz), 7.39 (2H, s, N−
H), 7.58 (1H, dd, phen, JH−H = 8.0 and 5.6 Hz), 7.79 (1H, dd, phen,
JH−H = 5.2 and 0.8 Hz), 8.07 (3H, m, phen), 8.65 (1H, dd, phen, JH−H
= 8.0 and 0.8 Hz), 8.74 (1H, dd, phen, JH−H = 4.2 and 1.5 Hz), 9.37
(1H, dd, phen, JH−H = 4.0 and 1.2 Hz). MS-ESI (m/z). Obsd (calcd):
323.63 (323.63), [Pt(phen)(pip2NCNH)]2+; 341.62 (341.62), [Pt-
1H NMR spectra were recorded at room temperature using a Bruker
AC 400 MHz instrument. Deuterated solvents, CDCl3 [0.03% (v/v)
tetramethylsilane (TMS)], CD2Cl2 [0.05% (v/v) TMS], and CD3CN,
were purchased from Cambridge Isotope Laboratories. Spectra are
reported in parts per million (ppm) relative to TMS (δ = 0 ppm) for
CDCl3 and CD2Cl2 or relative to the protic solvent impurity in the
case of CD3CN (δ = 1.94 ppm for CD2HN). Elemental analysis was
performed by Atlantic Microlab, Inc. (Norcross, GA). UV−visible
absorption spectra were recorded using a HP8453 UV−visible diode-
array spectrometer. Emission spectra were recorded using a SPEX
Fluorolog-3 fluorimeter equipped with a double-emission mono-
chromator (385 nm cutoff filter) and a single excitation mono-
chromator. Glassy solutions (77 K) were prepared by inserting a
quartz electron paramagnetic resonance tube containing a N-
butyronitrile solution of the respective complex into a quartz-tipped
(phen)(pip2NCNH2)Cl]2+
(pip2NCNH2)Cl](PF6)+.
; 828.20 (828.20), [Pt(phen)-
[Pt(phen)(pip2NCNH2)Br](PF6)2 (2). The product was isolated as
a bright-yellow solid following the same procedure as that for 1 and
1
substituting KBr (12 mg, 0.1 mmol) for KCl. Yield: 0.066 g, 64%. H
NMR (CD3CN, δ): 1.47−1.77 (m, aliphatic pip2NCNH2 overlapping
with solvent resonances), 2.87 (2H, dd, aliphatic pip2NCNH2 α-H,
JH−H = 24.8 and 12.8 Hz), 3.04 (2H, dd, aliphatic pip2NCNH2 α-H,
JH−H = 25.6 and 10.4 Hz), 3.32 (2H, dd, aliphatic pip2NCNH2 α-H,
JH−H = 12.4 and 4.0 Hz), 3.62 (2H, dd, aliphatic pip2NCNH2 α-H,
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JH−H = 12.4 and 4.0 Hz), 4.18 (2H, dd, benzylic CH2, JH−H = 13.6 and
6.0 Hz), 4.91 (2H, dd, benzylic CH2, JH−H = 12.4 and 1.6 Hz), 7.27
(1H, t, aromatic pip2NCNH2, JH−H = 5.2 Hz), 7.41 (2H, d, aromatic
pip2NCNH2, JH−H = 5.2 Hz), 7.47 (2H, s, N−H), 7.69 (1H, dd, phen,
3.04 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 14.0 and 10.0 Hz),
3.31 (2H, d, aliphatic pip2NCNH2 α-H, JH−H = 12.8 Hz), 3.71 (2H, d,
aliphatic pip2NCNH2 α-H, JH−H = 12.0 Hz), 4.26 (2H, dd, benzylic
CH2, JH−H = 12.8 and 0.4 Hz), 4.98 (2H, dd, benzylic CH2, JH−H
=
JH−H = 8.0 and 6.48 Hz), 7.88 (1H, dd with Pt satellites, phen, JH−Pt
∼
13.2 and 0.4 Hz), 7.35 (1H, dd, aromatic pip2NCNH2, JH−H = 8.4 and
6.8 Hz), 7.45 (2H, d, aromatic pip2NCNH2, JH−H = 7.6 Hz), 7.70 (1H,
dd, phen, JH−H = 8.0 and 5.6 Hz), 7.84 (1H, dd with Pt satellites, phen,
JH−Pt = 40 Hz, JH−H = 5.6 and 1.2 Hz), 8.17 (1H, dd, phen, JH−H = 8.4
and 5.2 Hz), 8.18 (2H, bs, N−H), 8.20 (1H, d, phen, JH−H = 9.2 Hz),
8.27 (1H, d, phen, JH−H = 9.2 Hz), 8.80 (1H, dd, phen, JH−H = 5.6 and
1.2 Hz), 8.81 (1H, d, phen, JH−H = 2.0 and 1.6 Hz), 8.97 (1H, dd,
phen, JH−H = 8.0 and 1.2 Hz). MS-ESI (m/z). Obsd (calcd): 347.12
(347.13), [Pt(phen)(pip2NCNH2)(NO2)]2+; 693.25 (693.25), [Pt-
(phen)(pip2NCNH)(NO2)]+; 839.22 (839.22), [Pt(phen)-
(pip2NCNH2)(NO2)](PF6)+.
60 Hz, JH−H = 5.6 and 1.2 Hz), 8.20 (2H, t, phen, JH−H = 8.4 Hz), 8.23
(1H, dd, phen, JH−H = 8.0 and 1.2 Hz), 8.83 (1H, dd, phen, JH−H = 8.0
and 1.2 Hz), 8.91 (1H, dd, phen, JH−H = 8.4 and 1.2 Hz), 9.68 (1H, dd,
1
phen, JH−H = 4.8 and 1.6 Hz). H NMR (CD2Cl2, δ): 0.59−1.98 (m
overlapping with solvent resonances, aliphatic pip2NCNH2), 2.85 (2H,
dd, aliphatic pip2NCNH2 α-H, JH−H = 25.2 and 12.0 Hz), 3.09 (2H,
dd, aliphatic pip2NCNH2 α-H, JH−H = 26.0 and 10.8 Hz), 3.37 (2H,
dd, aliphatic pip2NCNH2 α-H, JH−H = 12.0 and 4.4 Hz), 3.65 (2H, dd,
aliphatic pip2NCNH2 α-H, JH−H = 12.2 and 2.0 Hz), 4.11 (2H, dd,
benzylic CH2, JH−H = 12.8 and 10.8 Hz), 5.03 (2H, dd, benzylic CH2,
JH−H = 12.4 and 0.8 Hz), 7.24 (1H, t, aromatic pip2NCNH2, JH−H = 8.0
Hz), 7.37 (2H, d, aromatic pip2NCNH2, JH−H = 8.0 Hz), 7.42 (2H, s,
N−H), 7.60 (1H, dd, phen, JH−H = 9.2 and 5.6 Hz), 7.72 (1H, dd,
phen, JH−H = 4.8 and 2.4 Hz), 8.08 (2H, phen, m), 8.67 (1H, dd, phen,
JH−H = 8.0 and 1.2 Hz), 8.73 (1H, dd, phen, JH−H = 8.0 and 2.0 Hz),
9.57 (1H, d, phen, JH−H = 6.0 Hz). MS-ESI (m/z). Obsd (calcd):
323.63 (323.63), [Pt(phen)(pip2NCNH)]2+; 364.59 (364.59), [Pt-
[Pt(NO2phen)(pip2NCNH2)Cl](PF6)2 (7). The product was iso-
lated as a lemon-yellow solid following the same procedure as that for
1 and substituting 5-nitro-1,10-phenanthroline (NO2phen; 23 mg, 0.1
1
mmol) for phen. Yield: 0.065 g, 64%. H NMR (CD3CN, δ): 1.47
(2H, m, aliphatic pip2NCNH2), 1.61 (2H, m, aliphatic pip2NCNH2),
1.71−1.94 (m, aliphatic pip2NCNH2 overlapping with solvent
resonances), 2.87 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 10.0
and 4.0 Hz), 3.06 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 9.2 and
5.6 Hz), 3.29 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 9.2 and 5.6
Hz), 3.66 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 12.4 and 2.4
Hz), 4.16 (2H, dd, benzylic CH2, JH−H = 17.6 and 4.8 Hz), 4.94 (2H,
dd, benzylic CH2, JH−H = 11.6 and 4.8 Hz), 7.31 (1H, t, aromatic
pip2NCNH2, JH−H = 7.2 Hz), 7.41 (2H, d, aromatic pip2NCNH2, JH−H
= 6.8 Hz), 7.47 (2H, bs, N−H), 7.78 (1H, dd, NO2phen, JH−H = 13.6
and 4.8 Hz), 8.08 (1H, dd with Pt satellites, NO2phen, JH−Pt = 56 Hz,
JH−H = 9.6 and 1.2 Hz), 8.38 (1H, d, NO2phen, JH−H = 6.0 Hz), 9.01
(1H, d, NO2phen, JH−H = 8.0 Hz), 9.15 (1H, s, NO2phen), 9.46 (1H,
d, NO2phen, JH−H = 8.0 Hz), 9.63 (1H, dd, JH−H = 9.6 and 1.2 Hz).
MS-ESI (m/z). Obsd (calcd): 363.61 (363.61), [Pt(NO2phen)-
(pip2NCNH2)Cl]2+; 874.18 (874.18), [Pt(NO2phen)(pip2NCNH2)-
Cl](PF6)+.
[Pt(dtfmbpy)(pip2NCNH2)Cl](PF6)2 (8). The product was isolated
as a bright-yellow solid following the same procedure as that for 1 and
substituting dtfmbpy (30 mg, ∼0.1 mmol) for phen. Yield: 0.060 g,
55%. 1H NMR spectra showed two sets of closely overlapping
resonances in a 3:2 intensity ratio. 1H NMR (CD3CN, δ): 1.54 (2.4H,
m, aliphatic pip2NCNH2), 1.62 (1.6H, m, aliphatic pip2NCNH2),
1.77−2.10 (m, aliphatic pip2NCNH2 overlapping with solvent
resonances), 2.75 (0.8H, dd, aliphatic pip2NCNH2, JH−H = 12.4 and
2.4 Hz), 2.88 (1.2H, dd, aliphatic pip2NCNH2, JH−H = 12.4 and 2.4
Hz), 3.11−3.24 (4H, m, aliphatic pip2NCNH2), 3.72 (2H, dd, aliphatic
pip2NCNH2), 3.87 (0.8H, dd, benzylic CH2, JH−H = 13.6 and 4.8 Hz),
4.20 (1.2H, dd, benzylic CH2, JH−H = 13.6 and 4.8 Hz), 4.60 (0.8H, dd,
benzylic CH2, JH−H = 11.6 and 4.8 Hz), 5.00 (1.2H, dd, benzylic CH2,
JH−H = 11.6 and 4.8 Hz), 6.91 (0.6H, dd, JH−H = 9.6 and 4.8 Hz), 6.97
(0.4H, dd, JH−H = 9.6 and 4.8 Hz), 7.35−7.44 (5H, m, aromatic
protons overlapping with N−H resonances), 7.57 (1.2H, m), 7.75
(0.8H, m), 8.64−8.74 (2H, m), 9.52 (0.6H, d, JH−H = 8.0 Hz), 9.55
(0.4H, d, JH−H = 8.0 Hz). MS-ESI (m/z). Obsd (calcd): 397.11
(397.10), [Pt(dtfmbpy)(pip2NCNH2)Cl]2+; 941.18 (941.18), [Pt-
(dtfmbpy)(pip2NCNH2)Cl](PF6)+.
(phen)(pip2NCNH2)Br]2+
; 874.15 (874.15), [Pt(phen)-
(pip2NCNH2)Br](PF6)+.
[Pt(phen)(pip2NCNH2)I](PF6)2 (3). The product was isolated as a
bright-yellow solid following the same procedure as that for 1,
substituting KI (17 mg, ∼0.1 mmol) for KCl, and stirring the solution
at room temperature overnight instead of heating. Yield: 0.070 g, 66%.
1H NMR (CD3CN, δ): 1.42 (2H, m), 1.60−1.86 (m, aliphatic
pip2NCNH2 overlapping with solvent resonances), 2.83 (2H, dd,
aliphatic pip2NCNH2 α-H, JH−H = 24.4 and 11.2 Hz), 3.00 (2H, dd,
aliphatic pip2NCNH2 α-H, JH−H = 22.0 and 11.2 Hz), 3.33 (2H, dd,
aliphatic pip2NCNH2 α-H, JH−H = 12.4 and 2.4 Hz), 3.52 (2H, dd,
aliphatic pip2NCNH2 α-H, JH−H = 12.0 and 2.8 Hz), 4.35 (2H, dd,
benzylic CH2, JH−H = 13.2 and 8.0 Hz), 4.75 (2H, dd, benzylic CH2,
JH−H = 13.2 and 1.2 Hz), 7.25 (1H, t, aromatic pip2NCNH2, JH−H = 7.6
Hz), 7.39 (2H, d, aromatic pip2NCNH2, JH−H = 7.6 Hz), 7.53 (2H, s,
N−H), 7.75 (1H, dd, phen, JH−H = 8.8 and 5.6 Hz), 7.87 (1H, dd,
phen, JH−H = 6.8 and 1.2 Hz), 8.18 (1H, dd with Pt satellites, phen,
JH−Pt = 62 Hz, JH−H = 8.8 and 4.4 Hz), 8.23 (2H, t, phen, JH−H = 6.8
Hz), 8.89 (2H, dd, phen, JH−H = 4.4 and 2.0 Hz), 10.03 (1H, dd, phen,
JH−H = 5.2 and 1.6 Hz). MS-ESI (m/z). Obsd (calcd): 387.57
(387.59), [Pt(phen)(pip2NCNH2)I]2+; 774.17 (774.16), [Pt(phen)-
(pip2NCNH)I]+; 920.14 (920.14), [Pt(phen)(pip2NCNH2)I](PF6)+.
[Pt(phen)(pip2NCNH2)(NCS)](PF6)2 (4). The product was isolated
as a pale-yellow solid following the same procedure as that for 3 and
substituting KSCN (10 mg, ∼0.1 mmol) for KI. Yield: 0.070 g, 70%.
1H NMR (CD3CN, δ): 0.88 (2H, m, aliphatic pip2NCNH2), 1.42−
2.02 (m, aliphatic pip2NCNH2 overlapping with solvent resonances),
2.86 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 14.4 and 12.4 Hz),
3.05 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 14.4 and 12.4 Hz),
3.32 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 12.4 and 2.4 Hz),
3.64 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 12.4 and 2.4 Hz),
4.14 (2H, dd, benzylic CH2, JH−H = 12.8 and 2.0 Hz), 4.92 (2H, dd,
benzylic CH2, JH−H = 12.8 and 2 Hz), 7.29 (2H, d, aromatic
pip2NCNH2, JH−H = 7.6 Hz), 7.38 (1H, t, aromatic pip2NCNH2, JH−H
= 7.2 Hz), 7.59 (1H, d, phen, JH−H = 7.6 Hz), 7.66 (1H, d, phen, JH−H
= 5.6 Hz), 7.92 (1H, dd, phen, JH−H = 4.8 and 2.0 Hz), 8.22 (1H, t,
phen, JH−H = 7.6 Hz), 8.24 (1H, t, phen, JH−H = 8.0 Hz), 8.21 (2H, bs,
N−H), 8.80 (1H, dd, phen, JH−H = 4.8 and 2.0 Hz), 8.92 (1H, dd,
phen, JH−H = 4.4 and 2.0 Hz), 9.49 (1H, dd, phen, JH−H = 8.0 and 1.2
Hz). MS-ESI (m/z). Obsd (calcd): 353.12(353.12), [Pt(phen)-
(pip2NCNH2)(NCS)]2+; 705.24 (705.23), [Pt(phen)(pip2NCNH)-
(NCS)]+.
Pt(pipNC)(DMSO)Cl. A solution of sodium acetate (0.32 g, 2.4
mmol), pipNCH (0.415 g, 2.4 mmol), and Pt(DMSO)2Cl2 (1.0 g, 2.4
mmol) in 75 mL of methanol was stirred for 4 days at room
temperature. After rotary evaporation, the residue was treated with
water (25 mL) and extracted with CH2Cl2 (3 × 50 mL). The
combined CH2Cl2 layers were dried over MgSO4, filtered, and rotary
evaporated to dryness. The residue was purified by column
chromatography (silica: 10 in. length, 1.25 in. diameter, 19:1
CHCl3/acetone). The product was further purified by vapor diffusion
of diethyl ether into a methylene chloride solution to give an off-white
solid. Yield: 0.44 g, 39%. Anal. Calcd for (C12H16N)(C2H6SO)PtCl: C,
[Pt(phen)(pip2NCNH2)(NO2)](PF6)2 (5). The product was isolated
as a pale-yellow solid following the same procedure as that for 1 and
substituting KNO2 (9 mg, ∼0.1 mmol) for KCl. Yield: 0.070 g, 71%.
1H NMR (CD3CN, δ): 1.47 (2H, m, aliphatic pip2NCNH2), 1.66 (2H,
m, aliphatic pip2NCNH2), 1.78−1.91 (8H, m, aliphatic pip2NCNH2),
2.88 (2H, dd, aliphatic pip2NCNH2 α-H, JH−H = 14.4 and 12.4 Hz),
1
34.82; H, 4.59; N, 2.90. Found: C, 34.57; H, 4.52; N, 2.92. H NMR
(CDCl3, δ): 1.42−1.92 (6H, m, CH2), 3.13 (2H, m, CH2), 3.55 (6H, s
with Pt satellites, JH−Pt = 25 Hz, CH3), 3.92 (2H, m, CH2), 4.28 (2H, s
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a
Scheme 2. Synthesis of Pt(diimine)(pip2NCNH2)(L)2+
a
n
(i) Piperidine/benzene; (ii) BuLi, Pt(COD)Cl2/THF, −70 °C; (iii−v) TFA, diimine, KL/CH3CN.
with Pt satellites, JH−Pt = 45 Hz, CH2), 6.99−7.13 (3H, m, CH), 7.92
(2H, d with Pt satellites, JH−Pt = 47 Hz, CH).
X-ray Crystallography. Single crystals of the hexafluorophosphate
salts of Pt(phen)(pip2NCNH2)(L)2+ [L = Cl (1), Br (2), I (3), NCS
(4), NO2 (5), and Pt(dtfmbpy)(pip2NCNH2)Cl2+ (8)], as well as the
BF4− salt of Pt(phen)(pipNHC)Cl+ (9), were obtained from CH3CN/
Et2O. In the case of Pt(phen)(pip2NCNH2)Cl2+, a crystal form
containing CH3CN and H2O solvate also was obtained as pale-yellow
blocks from CH3CN/CH2Cl2/Et2O (1·0.25CH3CN·0.5H2O). In the
case of crystals containing [Pt(phen)(pip2NCNH2)(OCN)](PF)2 (6),
a yellow solid was isolated by following the same procedure as that for
1 and substituting KNCO (8 mg, ∼0.1 mmol) for KCl; single crystals
were obtained by vapor diffusion of Et2O into a CH3CN solution of
the product. Single crystals of 7·0.25CH3OH were obtained from
CH3CN/Et2O/CH3OH. For X-ray examination and data collection,
suitable crystals of appropriate dimensions were mounted in a loop
with paratone-N and transferred immediately to the goniostat bathed
in a cold stream.
Low-temperature intensity data were collected using synchrotron
radiation (λ = 0.775 Å) with either a Bruker Platinum 200 or APEX2
CCD detector at Beamline 11.3.1 at the Advanced Light Source
(Lawrence Berkeley National Laboratory) or using Cu Kα radiation (λ
= 1.54178 Å) with a Bruker SMART6000 CCD diffractometer. Data
frames were collected using the APEX2 or SMART programs,
respectively, and processed using SAINT. The data were corrected
for absorption and beam corrections based on the multiscan technique
as implemented in SADABS. The structures were solved by a
combination of either direct methods or the Patterson method using
SHELXTL or SIR2004, expanded using the difference Fourier
technique, and refined by full-matrix least squares on F2. Non-H
atoms were refined with anisotropic displacement parameters with the
exception of the disordered F atoms in 2 and the solvate in
7·0.25CH3OH. The NH piperidinium and water solvate H atoms were
located directly from the difference map and held fixed at that location
in subsequent refinement cycles; the one exception is
1·0.25CH3CN·0.5H2O, for which the position of one water H atom
was calculated based on hydrogen-bonding interactions. The
remaining H-atom positions were calculated and treated with a riding
model; isotropic displacement parameters were defined as aUeq of the
adjacent atom (a = 1.5 for −CH3, OH, and solvate atoms and 1.2 for
all others). The PF6− anions show typical disorder in these complexes,
[Pt(phen)(pipNHC)Cl](BF4) (9). The product was isolated as a
pale-yellow solid following the same procedure as that for
[(pip2NCNH2)Pt(phen)Cl](PF6)2 and substituting Pt(pipNC)-
(DMSO)Cl (49 mg, ∼0.1 mmol) for Pt(pip2NCN)Cl and AgBF4
(19 mg, ∼0.1 mmol) for AgPF6. Yield: 0.035 g, 52%. 1H NMR
(CD3CN, δ): 1.43 (2H, m), 1.61 (2H, m), 1.77 (4H, m, aliphatic
pipNHC), 1.87 (4H, m, aliphatic pipNHC), 2.85 (1H, dd, aliphatic
pipNHC α-H, JH−H = 14.4 and 12.0 Hz), 3.00 (1H, dd, aliphatic
pipNHC α-H, JH−H = 14.8 and 12.0 Hz), 3.30 (1H, dd, aliphatic
pipNHC α-H, JH−H = 11.6 and 2.0 Hz), 3.61 (1H, dd, aliphatic
pipNHC α-H, JH−H = 10.0 and 2.0 Hz), 4.04 (1H, dd, benzylic CH2,
JH−H = 12.8 and 2.0 Hz), 4.70 (1H, dd, benzylic CH2, JH−H = 12.0 and
2.0 Hz), 7.13 (1H, t, aromatic pipNHC, JH−H = 8.0 Hz), 7.23 (2H, d,
aromatic pipNHC, JH−H = 8.0 Hz), 7.49 (1H, d, phen, JH−H = 6.8 Hz),
7.68 (1H, d, phen, JH−H = 6.8 Hz), 8.05 (1H, dd with Pt satellites,
aromatic pipNHC, JH−Pt = 56 Hz, JH−H = 6.8 and 1.2 Hz), 8.24 (2H, t,
phen, JH−H = 6.0 Hz), 8.32 (1H, t, phen, JH−H = 6.8 Hz), 8.35 (1H, s,
N−H), 8.78 (1H, dd, phen, JH−H = 8.4 and 0.8 Hz), 8.85 (1H, dd,
phen, JH−H = 8.4 and 0.8 Hz), 9.54 (1H, dd, phen, JH−H = 6.8 and 0.8
Hz). MS-ESI (m/z). Obsd (calcd): 585.15 (585.14), [(pipNHC)Pt-
(phen)Cl]+; 637.17 (637.17), [(pipNHC)Pt(phen)](BF4)+.
Pt(phen)(pip2NCN)Cl. The complex was generated in situ by
adding 2 equiv of KOH (1.0 mM in CD3OD) to a 0.5 mM solution of
1 in CD2Cl2 or by adding 2 equiv of TBAOH (1.0 mM) to a 0.5 mM
1
solution of 1 in CD3CN. Excess base resulted in decomposition. H
NMR (CD2Cl2, δ): 0.75−3.70 (m, aliphatic pip2NCN overlapping
with solvent resonances), 4.11 (2H, d, benzylic CH2), 4.14 (2H, d,
benzylic CH2), 6.69 (2H, d, aromatic pip2NCN, JH−H = 6.8 Hz), 6.81
(1H, t, aromatic pip2NCN, JH−H = 7.6 Hz), 7.61 (2H, dd, phen, JH−H
=
8.0 and 4.4 Hz), 7.78 (2H, s, phen), 8.25 (2H, d, phen, JH−H = 8.0 Hz),
1
9.04 (2H, d, phen, JH−H = 3.6 Hz). H NMR (CD3CN, δ): 0.8−4.0
(m, aliphatic pip2NCN overlapping solvent resonances and resonances
from TBA+ cation of the base), 4.32 (bs, benzylic CH2), 7.12 (4H, m,
aromatic pip2NCN overlapping with phen resonance), 7.65 (1H, m,
phen), 8.17 (3H, m, phen), 8.77 (1H, t, phen, JH−H = 8.8 Hz), 8.86
(1H, t, phen, JH−H = 8.4 Hz), 9.64 (1H, m, phen).
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a
Scheme 3. Synthesis of Pt(diimine)(pipNHC)Cl+
a
(i) Piperidine/benzene; reflux; (ii) Pt(DMSO)2Cl2, NaOAC/CH3OH; (iii) TFA, 1,10-phenanthroline, KCl/CH3CN.
which was described with two- or three-component disorder models
when possible. In several cases, it was necessary to apply bond
restraints or constrain the displacement parameters to keep the anion
at a reasonable geometry.
pattern of resonances. Notably, for each complex, one set of
diimine resonances and two phenyl proton resonances (in an
intensity ratio of 2:1) are observed in the aromatic region. For
example, in CD3CN, the benzylic proton resonances appear in
an AX pattern near 4.2 and 4.9 ppm, establishing that these
protons are diastereotopic and that coupling to the amine
proton is weak, as noted for related systems.29 For each
complex, the chemical shift difference between the benzylic
proton resonances is 0.7−0.8 ppm (except for the iodide
complex, where it is 0.4 ppm), suggesting significantly different
chemical environments. For the series of complexes, the α-
piperidinium protons give rise to four resonances with equal
intensities from 2.8 to 3.7 ppm; the β- and γ-proton resonances
overlap with the solvent and water signals in most cases. Taken
together, these data confirm that the combination of inversion
about the piperidyl N atom and inversion of the piperidyl ring
is slow on the NMR time scale, as expected for a protonated
piperidyl group. In addition, asynchronous rotation about the
Pt−C bond must also be slow, which is consistent with steric
For crystals containing Pt(phen)(pip2NCNH2)(OCN)2+, the
structure was refined with both an OCN group and a Cl atom
bonded to the Pt atom with occupancies of 75% and 25%, respectively,
giving the most reasonable anisotropic displacement parameters, bond
distances, and bond angles. Attempts at modeling the −OCN group
such that the N atom is bonded to Pt resulted in very unfavorable
anisotropic displacement parameters, bond distances, and bond angles.
For crystals containing Pt(phen)(pip2NCNH2)I2+, a suitable multi-
component disorder model for a few of the C atoms in one of the
piperidinium groups was not realized. In crystals with fractional
solvate, the solvate was disordered, and restraints were used in some
cases.
RESULTS AND DISCUSSION
■
Synthesis. The rational design and synthesis of square-
planar metal complexes with more than four nucleophiles
capable of interacting with the metal center is facilitated by
using protonation to protect the more basic dangling groups
and prevent the formation of a coordinate bond. Scheme 2
shows an efficient synthetic route to arylplatinum(II)
complexes, Pt(diimine)(pip2NCNH2)(L)2+, with a chelating
diimine ligand (phen, NO2phen, or dtfmbpy), a monodentate
anionic ligand (L = Cl, Br, I, NCS, NO2, or OCN), and two
dangling protonated piperidyl groups. The chloride ligand was
removed from Pt(pip2NCN)Cl by reaction with AgPF6. A
subsequent reaction with the diimine and KL in the presence of
trifluoroacetic acid gave [Pt(diimine)(pip2NCNH2)(L)](PF6)2
in 50−70% yield. Excess KSCN and KI were avoided in order
to prevent the formation of Pt(diimine)(SCN)2 and Pt-
(diimine)I2, respectively. In order to synthesize a complex
with one dangling piperidinium group using the pipNC− ligand,
we sought to prepare a precursor complex with a single
chelating pipNC− ligand. Reactions of the pipNC− ligand with
Pt(COD)Cl2 proved unsuccessful, resulting in the formation of
Pt(pipNC)2, which appears to be a thermodynamic sink in this
chemistry.29 In an alternative approach, Pt(DMSO)2Cl2,
pipNCH, and sodium acetate were refluxed in methanol for 4
h to yield Pt(pipNC)(DMSO)Cl. Treatment with AgBF4 and
the subsequent reaction with phen and KCl under acidic
conditions gave 9 in ∼50% yield (Scheme 3).
considerations.29,38−41 Interestingly, the H NMR spectra of
1
the Pt(phen)(pip2NCNH2)X2+ (X = Cl, Br, or I) series reveals
that the furthest downfield resonance, assigned to a phen α
proton, shifts downfield along the series Cl < Br < I (9.48, 9.68,
and 10.03 ppm, respectively), varying linearly with Gutman’s
donor number for the corresponding halide.42
For each complex, the piperidinium NH protons give rise to
a broadened resonance in the 7.4−8.2 ppm range. The chemical
shifts are similar to those observed for Pt(diimine)-
2+
(pipNHC)2 complexes, which show evidence of NH···Pt
hydrogen bonding in solution and the solid state.29 On the
other hand, this resonance is shifted significantly downfield
from that of Pt(tpy)(pip2NCNH2)3+ (6.88 ppm).26 The Pt
center in the latter complex is expected to be comparatively
electron-poor, resulting in weaker interactions with the
dangling nucleophilic piperidinium groups. Downfield chemical
shifts of amine proton resonances from free ligand values are a
signature of NH···Pt hydrogen bonding43,44 but are by no
means conclusive proof thereof.45 For example, we note that
the amine resonance shows no evidence of 195Pt satellites and
these NMR data alternatively could be interpreted as being
consistent with the notion of NH···L hydrogen bonding
involving the halide or pseudohalide group (L−) that is bonded
to the metal. In support of this possibility, we note that in their
detailed studies of the zwitterionic compounds A−D depicted
in Scheme 4, van Koten and co-workers46−48 encountered
examples of both NH···Pt and NH···Br interactions, where the
Br was bonded to the Pt center. In CDCl3, complex A with the
rigid protonated naphthyl ligand (1-C10H6N(H)Me2-8-C,H)
exhibited the strongest NH···Pt hydrogen-bonding interactions,
1
The products were characterized using H NMR spectros-
copy, MS, and X-ray crystallography. The protonated
complexes are stable in an acetonitrile solution and can be
treated with 2 equiv of base to give deprotonated adducts. The
latter are stable in methylene chloride but gradually decompose
in acetonitrile to give the respective Pt(pip2NCN)Ln+
complexes (L = halide/pseudohalide or solvent).
1
characterized by amine H NMR resonances near 16 ppm and
195Pt coupling constants, J(195Pt,1H), in the 150−180 Hz range.
Such interactions are sufficient to prevent the oxidative addition
of HCl to the bis-chelate parent complex, Pt(1-C10H6NMe2-8-
1
1H NMR Spectroscopy. The room-temperature H NMR
spectra of the diprotonated complexes with phenanthrolinyl
ligands in CD3CN and CD2Cl2 solutions show the expected
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remaining bonded to Pt. The benzylic resonances appear as a
broad singlet at 4.32 ppm, which is suggestive of monodentate
pip2NCN− and comparatively rapid rotation about the Pt−C
bond. Upon standing, the Pt(phen)(pip2NCN)Cl resonances
gradually lose intensity, while new resonances associated with
Pt(pip2NCN)Cl and phen gain intensity, indicating that the
deprotonated product is unstable. Under typical conditions
(∼1.0 mM), the first-order rate constant is in the (2−4) × 10−4
s−1 range, which is somewhat less than that found for the
Pt(phen)(pipNC)2 analogue (10−3 s−1) but slightly greater
than that of Pt(tpy)(pip2NCN)+ (10−4 s−1). Thus, the stability
for this series of complexes, namely, Pt(phen)(pipNC)2,
Pt(phen)(pip2NCN)Cl, and Pt(tpy)(pip2NCN)+ (Scheme 1),
increases with increasing rigidity of the ligand framework.
Crystal Structures. The structures of eight diprotonated
complexes, Pt(phen)(pip2NCNH2)(L)2+ (L = Cl, Br, I, NCS,
NO2, or OCN), Pt(dtfmbpy)(pip2NCNH2)Cl2+, and Pt-
(NO2phen)(pip2NCNH2)Cl+, and one monoprotonated com-
plex, Pt(phen)(pipNHC)Cl+, were confirmed by X-ray
crystallography (Figure 1 and Tables 1−4). The structure of
1 was determined for crystals without and with solvate
(1·0.25CH3CN·0.5H2O; Figure S1 in the Supporting Informa-
tion). An attempt to grow crystals of the cyanate complex,
Pt(phen)(pip2NCNH2)(OCN)2+, yielded a crystal whose
diffraction pattern was best modeled as Pt(phen)-
(pip2NCNH2)(L)2+ (L = OCN or Cl) with occupancies of
75% and 25% for O-bonded OCN and Cl, respectively. To our
knowledge, this is the first example of a structurally
characterized platinum(II) O-bonded cyanate complex. It
should be noted that the NH piperidinium and water solvate
H atoms were located directly from the difference maps. The
one exception is 1·0.25CH3CN·0.5H2O, for which the position
of one water H atom was calculated based on hydrogen-
bonding interactions. The positions of all other H atoms were
calculated. In each case, the molecules pack as discrete units,
and there are no unusually short interionic interactions.
However, in crystals containing Pt(phen)(pip2NCNH2)-
(NCS)2+, the water solvate participates in hydrogen bonds
with one protonated piperidyl group [N4···O1W = 2.794(4) Å]
and the acetonitrile solvate [OW1···N7 = 2.847(5) Å; Figure S1
in the Supporting Information]. In crystals of
1·0.25CH3CN·0.5H2O, the water solvate forms a short
hydrogen bond with the chloro ligand [O1···Cl = 2.419(6)
Å; Figure S1 in the Supporting Information] and a longer
interaction with the acetonitrile solvate [O1···N5 = 2.886(24)
Å]. On the other hand, for [Pt(NO2phen)(pip2NCNH2)Cl]2+,
the −OH group of methanol solvate forms a weak interaction
with the chloro ligand [O1S···Cl = 3.675(1) Å; Figure S1 in the
Supporting Information].
Scheme 4. (A) Pt(1-C10H6NMe2-8-C,N)(1-C10H6NHMe2-8-
C,H)X, (B) Pt(C6H4CH2NR2)(C6H4CH2NHR2)Br (R = Me
and Et), and (C and D) Pt[(R)-
C6H4CH(Me)N(CH3)2][(R)-
C6H4CH(Me)NH(CH3)2]Br46,47
C,N)2; on the other hand, CF3COOH oxidatively adds to form
the platinum(IV) hydride as a result of a proposed CF3COO−
neighboring group effect.49 In the case of complexes B and C
with the less rigid and comparatively flexible benzylic ligand
backbone (Scheme 4), the amine resonances are shifted upfield
(11−13 ppm) and exhibit smaller coupling constants (66−104
Hz). Two examples of A and B were found to exhibit cross
1
peaks in the 2D H NOESY NMR spectra that are consistent
with persistent NH···Pt interactions, and J(195Pt,1H) was
proposed to show a correlation with J(15N,1H) and ν(N−
H).46−48 However, in the case of compound D, which is a
conformational isomer of C, steric repulsion between the
benzylic methyl group and the −N(CH3)2 group was proposed
to weaken the NH···Pt interaction. Consistent with this
conclusion, the amine resonance at 10.9 ppm showed no
evidence of 195Pt coupling, which is similar to our observations
for the Pt(diimine)(pip2NCNH2)X2+ series. A crystal structure
of D shows the NH group directed toward the middle of the
Pt−Br bond, which led those authors to suggest the existence
of NH···Br interactions.46,47 Compared to those zwitterionic
complexes, the metal centers in the Pt(diimine)(pip2NCNH2)-
X2+ series are likely to be less basic and form weaker NH···Pt
interactions, and therefore NH···L interactions may reasonably
be expected to play a comparatively more prominent role.
Interestingly, attempts to prepare [Pt(dtfmbpy)-
(pip2NCNH2)Cl]2+ yielded two products that we were unable
1
to separate. Even H NMR spectra of dissolved crystals of 8
show two sets of resonances consistent with two similar
products. The aromatic and piperidyl resonances of two species
appear as very closely overlapping resonances. There are two
sets of benzylic proton resonances appearing as AX patterns
near 4.6 and 4.3 ppm, respectively, in a 3:2 intensity ratio. By
contrast, the crystal structure shows a single isomer (vide infra),
and the MS spectrum is dominated by two fragment peaks
consistent with [Pt(dtfmbpy)(pip2NCNH2)Cl](PF6)+ and
[Pt(dtfmbpy)(pip2NCNH2)Cl]2+.
The deprotonated Pt(phen)(pip2NCN)(L) complexes were
prepared in situ by the addition of 2 equiv of base. For example,
the addition of 2 equiv of TBAOH to a solution of
[Pt(phen)(pip2NCNH2)Cl]2+ in CD3CN causes the piperdi-
nium proton resonance to disappear. In freshly prepared
solutions, there is no evidence of a free phen ligand or
Pt(pip2NCN)Cl, which is consistent with the diimine ligand
For each complex, the coordination geometry is approx-
imately square-planar, with N−Pt−L and N−Pt−C bond angles
[171.5(1)−177.2(4)°] similar to those reported for Pt-
(diimine)(pipNHC)2 complexes [171.4(2)−174.3(1)°].29
2+
Overall, the bond lengths and angles of the cations of the
chloro complexes are similar to those of Pt(2,2′-bpy)(p-
tolyl)Cl.50 For each complex, the coordination plane (defined
by the four atoms bonded to Pt) shows a small tetrahedral
distortion. The largest root-mean-square deviations for the
diprotonated series are for the two complexes in the
asymmetric unit of the Pt(phen)(pip2NCNH2)I2+ salt (0.0742
and 0.0509 Å). The N2−Pt−L angles [96.46(13) and
96.62(13)°] also are slightly greater than those for the other
salts [92.9(4)−95.59(6)°, except the nitrito complex, which
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Figure 1. continued
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Figure 1. ORTEP drawings of cations in (a) 9·CH3CN, (b) 1, (c) 2, (d) 3, one of two independent molecules shown, (e) 4·CH3CN·H2O, (f)
5·CH3CN, (g) 7·0.25CH3OH, (h) 8, and (i) (1)0.25(6)0.75·0.5CH3CN. Anions, solvent, and H atoms [with the exception of those bonded to
N(piperidyl)] are omitted for clarity (50% probability ellipsoids).
Table 1. Crystal and Structure Refinement Data
1
1·0.25CH3CN·0.5H2O
[C30H37N4ClPt]
(PF6)2·0.25CH3CN·0.5H2O
173(2)
(1)0.25(6)0.75·0.5CH3CN
0.75{[C31H37N5OPt](PF6)2} +
0.25{[C30H37N4ClPt](PF6)2}·0.5CH3CN
150(2)
2
3
formula
[C30H37N4ClPt]
(PF6)2
[C30H37N4BrPt]
(PF6)2
[C30H37N4IPt]
(PF6)2
T, K
173(2)
173(2)
150(2)
cryst syst
space group
a, Å
monoclinic
C2/c
monoclinic
P2/c
monoclinic
P2/c
monoclinic
C2/c
orthorhombic
Pca21
23.6176(18)
10.6519(8)
27.906(2)
90
12.9652(8)
12.1867(8)
24.3433(15)
90
12.9207(4)
12.2183(4)
24.2249(8)
90
23.9210(19)
10.7075(8)
27.772(2)
90
24.886(3)
8.0909(10)
35.751(5)
90
b, Å
c, Å
α, deg
β, deg
γ, deg
98.797(2)
90
102.276(1)
90
102.173(2)
90
98.747(2)
90
90
90
R1/wR2 [I >
2σ(I)]
0.0302/0.0788
0.0346/0.0926
0.0343/0.0937
0.0351/0.0985
0.0375/0.0873
R1/wR2 (all
data)
0.0328/0.0807
0.0401/0.0959
0.0353/0.0946
0.0370/0.1000
0.0403/0.0886
was 96.5(2)°], which is suggestive of steric repulsion between
the I atom and nearby phen α-H atom. In each structure, the
aryl group accommodates the chelating diimine ligand by
rotation about the Pt−C bond, resulting in a dihedral angle
with the coordination plane in a narrow range [NCS−,
76.0(1)°; all others, 84.7(1)−88.4(1)°]. Hydrogen bonding
of a piperidinium group with solvate in the Pt(phen)-
(pip2NCNH2)(NCS)2+ salt causes the aryl group to be rotated
less. The Pt−N2(diimine) distances trans to the phenyl ring
[2.088(2)−2.113(5) Å] are very similar to those found for
Pt(diimine)(pipNHC)22+ complexes29 and fall within the range
observed for related complexes with aryl groups trans to
diimine ligands (2.08−2.12 Å).51−55 The Pt−N1 distances are
shorter [2.005(3)−2.043(5) Å], in accordance with the relative
trans influence of the halide/pseudohalide ligands. The Pt−N1
distances for the Pt(phen)(pip2NCNH2)(L)2+ structures follow
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Table 2. Crystal and Structure Refinement Data
4·2CH3CN·H2O
[C31H37N5SPt]
(PF6)2·2CH3CN·H2O
150(2)
triclinic
5·CH3CN
7·0.25CH3OH
[C30H36N5O2ClPt]
(PF6)2·0.25CH3OH
193(2)
triclinic
8
9·CH3CN
formula
[C30H37N5O2Pt]
(PF6)2·CH3CN
[C30H35N4F6ClPt]
(PF6)2
[C24H25N3ClPt]
BF4·CH3CN
193(2)
T, K
150(2)
150(2)
cryst syst
space group
a, Å
orthorhombic
Pna21
monoclinic
P21/c
monoclinic
P21/c
P1
̅
P1
̅
9.2003(6)
10.5437(3)
14.7258(4)
16.3354(5)
63.216(1)
16.174(3)
18.447(3)
12.683(2)
90
12.3801(4)
12.4952(4)
24.1575(8)
90
9.3658(7)
26.227(2)
11.0318(9)
90
b, Å
13.1174(9)
16.0170(11)
86.300(1)
c, Å
α, deg
β, deg
γ, deg
71.705(1)
90
79.461(1)
96.102(1)
90
104.003(2)
90
69.919(1)
90
72.667(1)
R1/wR2 [I >
2σ(I)]
0.0248/0.0615
0.0370/0.0905
0.0577/0.1546
0.0218/0.0535
0.0246/0.0621
R1/wR2 (all
data)
0.0263/0.0625
0.0399/0.0921
0.0587/0.1560
0.0234/0.0544
0.0303/0.0642
Table 3. Selected Distances for 1−3 (Å)
1
1·0.25CH3CN·0.5H2O
(1)0.25(6)0.75·0.5CH3CN
2
3
Pt−C13
Pt−N1
Pt−N2
Pt−L
H3···L
N3−L
H3···Pt
N3···Pt
H4···L
N4−L
H4···Pt
N4···Pt
2.014(3)
2.005(3)
2.103(3)
2.3103(8)
2.51
2.028(4)
2.011(4)
2.101(4)
2.3468(15)
2.77
2.031(4)
2.020(4)
2.019(3)
2.105(3)
2.4247(4)
2.58
1.994(6), 2.014(6)
2.043(5), 2.025(5)
2.113(5), 2.113(5)
2.5996(5), 2.6022(5)
2.73, 2.84
2.008(4)
2.101(4)
2.110(16), 2.236(13)
2.79, 2.78
3.474(16), 3.472(11)
2.58
3.360(3)
2.75
3.516(4)
2.62
3.476(3)
2.71
3.582(5), 3.663(5)
3.28, 2.82
3.335(3)
2.35
3.352(4)
2.44
3.329(4)
3.357(4)
2.49
3.722(5), 3.458(5)
2.84, 2.75
2.16, 2.30
3.124(17)
2.62
3.254(3)
2.85
3.304(4)
2.78
3.357(3)
3.02
3.673(5), 3.594(5)
2.52, 3.30
3.486(3)
3.417(4)
3.393(4)
3.535(4)
3.384(5), 3.750(5)
Table 4. Selected Distances for 4−8 (Å)
4·2CH3CN.H2O
5·CH3CN
7·0.25CH3OH
8
9·CH3CN
Pt−C13
Pt−N1
Pt−N2
Pt−L
H3···L
N3−L
H3···Pt
N3···Pt
H4···L
N4−L
H4···Pt
N4···Pt
2.031(3)
2.022(2)
2.088(2)
2.008(3)
2.03
2.021(6)
2.022(4)
2.098(5)
2.061(4)
2.68
2.035(5)
2.026(4)
2.107(5)
2.3157(13)
2.58
2.024(3)
2.021(2)
2.108(2)
2.3019(6)
2.47
2.012(3)
2.008(3)
2.107(2)
2.3130(8)
2.36
2.976(4)
2.84
3.490(7)
3.10
3.374(5)
2.86
3.342(2)
2.94
3.228(3)
3.02
3.502(3)
3.517(4)
2.62
3.395(4)
2.45
3.560(2)
2.62
3.538(3)
3.467(6)
2.98
3.230(4)
2.78
3.431(2)
2.71
3.700(4)
3.466(4)
3.389(2)
the trend Cl < Br < NCS ≈ NO2 < I [L, dPt−N1: Cl, 2.005(3) for
1 and 2.011(4) for 1·0.25CH3CN·0.5H2O; Br, 2.019(3); I,
2.025(5) and 2.043(5); NCS, 2.022(2); NO2, 2.022(4)]. The
Pt−L distances are normal,56−62 except that the Pt−Cl bond is
elongated for the structure in which the chloro ligand forms a
hydrogen bond with water solvate [2.3468(15) vs 2.3019(6)−
2.3157(13) Å for the other chloro structures, except the
disordered cyanate structure]. The Pt−C distances [1.994(6)−
2.035(5) Å] are within the range observed for Pt(pip2NCN)-
(L)+ complexes with pyridyl groups positioned trans to the aryl
ligand (1.91−2.04 Å).26,27,35,51,63,64
pseudohalide ligand (NH···L for 20 measurements: 2.03−2.84
Å). These interactions are emphasized in Figures 2 and S2 in
the Supporting Information. The lone exception is Pt(phen)-
(pip2NCNH2)(NCS)2+, in which one of the protonated
piperidyl groups forms a hydrogen bond with water solvate,
as mentioned previously. In nearly all instances, the NH···L
distances also are less than the sum of the van der Waals
radii.65,66 One exception is Pt(phen)(pip2NCNH2)(NO2)2+, in
which there are, instead, very short NH···O contacts involving
the nitrite O atoms [N···O = 2.808(7) and 2.891(6) Å; NH···O
= 1.91−2.06 Å] and the NO2− group forms a 83.2(2)° dihedral
angle with the coordination plane (Figure S2 in the Supporting
Information). The other exception is for one of the
piperidinium groups in the disordered cyanate complex
In nearly every structure, both of the piperidinium NH
groups are directed approximately toward the coordination
plane, resulting in short contacts to the metal-bonded halide or
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Figure 2. ORTEP drawings of cations showing the NH···L and NH···Pt interactions (dashed lines) that are less than the sum of the van der Waals
radii for (a) 1, (b) 2, (c) molecule B in 3, and (d) 4·CH3CN·H2O. Anions, solvent, C atoms of the diimine ligand, and H atoms [with the exception
of those bonded to N(piperidyl)] are omitted for clarity (50% probability ellipsoids).
[N3···O5 = 3.474(16)Å; H3···O5 = 2.79 Å]. For each complex,
the orientation of the NH groups also results in short NH···Pt
distances (2.52−3.30 Å). About half of these distances (11 of
20) are less than the sum of the van der Waals radii.
Additionally, the NH···Pt (110−147°) and NH···L (132−178°)
bond angles are consistent with hydrogen bonding. The
connection between the NH···Pt distance and the N−H···Pt
angle is suggested by an approximate linear relationship (r2=
0.70; Figure 3a). A similar relationship exists between the N−
H···L angle and the difference between the NH···L distance
(dH−L) and the van der Waals radius (rvdw(L)) of the metal-
bonded atom of the halide or pseudohalide ligand (L−; r2 =
0.61; Figure 3b).
linear NH···Pt arrangement, which would be more optimal for a
dipole−monopole interaction. We believe that similar con-
straints exist for the systems presented here. However, it is
apparent that the NH···Pt interactions are longer for
Pt(diimine)(pip2NCNH2)(L)2+, presumably because of com-
petition from the halide/pseudohalide ligand. Moreover, the
NH···L interactions appear to be stronger than the NH···Pt
interactions, as suggested by the relative values of the N−H···L
and N−H···Pt angles. [For the purposes of this discussion,
values for Pt(phen)(pip2NCNH2)Cl2+ derived from the
structure of the disordered cyanate complex are excluded.] In
addition, each of the NH···Pt distances and half of the NH···L
interactions (10 of 20) exceed the distance between the
piperidinium proton and the Pt−L bond centroid by ≥0.1 Å.
Thus, the NH groups have the appearance of being directed
more toward the L atom or Pt−L bond rather than toward the
Pt atom. Although the proton-acceptor properties of metal-
bonded halides46,47 and PtII centers are well docu-
mented,29,43,45−47,67−70 we know of only one structurally
For the purposes of comparison, we consider the structures
2+
of four previously investigated Pt(diimine)(pipNHC)2
complexes with short NH···Pt contacts (2.32−2.51 Å) and
N−H···Pt angles in the 141−155° range, suggesting NH···Pt
hydrogen bonding.29 For those systems, it was proposed that
the geometric constraints of the pipNHC ligand prevent a more
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Figure 3. (a) N−H···Pt angle versus H···Pt distance: (light-green) chloro, (red) bromo and iodo, (blue) isocyanato, (dark-green) nitrito and
isothiocyanato, (solid line) linear fit to all data points [y = 235.9−37.7(d(H−Pt)); r2 = 0.70]. (b) N−H···Pt angle versus the difference between the
H···L distance (d(H−L)) and the van der Waals radius (rvdw(L)) of the metal-bonded atom of the halide or pseudohalide ligand: (light-green) chloro,
(red) bromo and iodo, (blue) isocyanato, (dark-green) nitrito and isothiocyanato, (solid line) linear fit to all data points (y = −51.6x + 199.4(d(H−L)
− rvdw(L)); r2 = 0.61).
characterized example in which the orientation of the N−H
vector is directed toward a Pt−L bond, namely, compound D
depicted in Scheme 4.46,47 In that case, steric repulsion between
the benzylic methyl group and the −N(CH3)2 group was
proposed to account for weakening of the NH···Pt interaction.
By contrast, for the series of structures reported here, there are
no obvious steric constraints favoring the observed orientation
of the N−H vector. Notably, even for the iodo complex, which
has the shortest NH···Pt contact (Figure S2b in the Supporting
Information), there is no evidence of increased steric
congestion. Moreover, the wide range of NH···Pt distances
(2.52−3.30 Å) and N−H···Pt angles (113.0−153.4°) also
suggests that steric/conformational factors are not overriding.
To understand the conformational changes contributing to
the variation in the NH···Pt contacts and N−H···Pt angles, we
have analyzed the structural variations in the 6-atom [···Pt−C−
C−C−N−H···] ring. To a good approximation, the Pt−C−C−
C fragment is planar, which reduces the dependence of the
NH···Pt distance to 11 conventional parameters (five bond
distances, four bond angles, and two bond torsion angles).
Accordingly, the calculated NH···Pt distances and N−H···Pt
angles based on these measured parameters are strongly
correlated with the experimental values (r2 = 0.94 and 0.98,
respectively; Figure S3 in the Supporting Information). The
experimental C−N−H angle and C−C−C−N torsion angle are
by far the most important parameters, and using average values
for all of the others still gives very good agreement between the
calculated and experimental values (r2: NH···Pt distances, 0.85;
N−H···Pt angles, 0.90; Figure S4 in the Supporting
Information). The importance of the C−N−H bond angle is
consistent with the rather wide variation in the experimental
values [average of 19 angles: 107(7)°]. The importance of
rotation about the benzylic C−C bond compared to rotation
about the C−N bond can be understood in terms of the arcs
swept out by the H atom upon rotation. The average radius for
the C−C rotation [1.95(8) Å] is more than twice that for the
C−N rotation [0.86(6) Å]. Additionally, the angle between the
Pt···H vector and a tangent to the arc of rotation at the H atom
tends to be greater for the benzylic C−C rotation [125(3) vs
100(11)°], making the Pt···H distance more sensitive to
rotation about the C−C bond.
We were especially interested in assessing the influence of
the electronic properties of the halide/pseudohalide ligand on
the NH···L/NH···Pt interactions. For this purpose, we define γ
= a + b as a measure of the combined NH···L and NH···Pt
interactions involving one piperidinium group, where r(H···L)
and r(H···Pt) are interatomic distances, rH, rPt, and rL are van
der Waals radii, and
⎧
⎪
0 if r(H···Pt) ≥ r + r
H
Pt
r(H···Pt) − r − r if r(H···Pt) < r + r
Pt
⎨
⎪
⎩
a =
b =
H
Pt
H
⎧
⎪
0 if r(H···L) ≥ r + r
H
L
⎨
⎪
⎩
r(H···L) − r − r if r(H···L) < r + r
L
H
L
H
Values of γ for 18 measurements range from −0.23 to −0.64 Å;
−
values for the NO2 complex (0.00, −0.02 Å) were excluded
because the N−H bond vectors are directed toward the nitrite
O atoms. Values of less than zero are indicative of significant
NH···L/NH···Pt interactions. For the complexes with thio-
cyanate (γ = −0.62 Å) and pipNHC (γ = −0.48 Å) ligands,
only one piperidinium group interacts with Pt/L. Values of γ
for these complexes are among the smallest, which is consistent
with a tendency for the removal of one piperidinium group to
be compensated for by strengthening the interactions with the
piperidinium group on the opposite face. For example, the
cyanate adduct has widely disparate values (−0.23, −0.64 Å).
Thus, the average of γ (⟨γ⟩) for the seven structures having two
interacting piperidinium groups falls within a very narrow range
(−0.35 to −0.48 Å). Interestingly, whereas ⟨γ⟩ varies only
modestly, the average N−H···X angle decreases along the
chloro (−0.44 Å, 177°), bromo (−0.45 Å, 171°), and iodo
(−0.44 Å, 158°) series of Pt(phen)(pip2NCNH2)(X)2+
complexes, in accordance with the relative gas-phase proton
affinities of the halides. It also has not escaped our notice that
⟨γ⟩ is larger and the average N−H···X angle ⟨θ⟩ is smaller for
the chloro adducts with more weakly donating diimine ligands
(NO2phen, −0.35 Å, 160°; dtfmbpy, −0.35 Å, 150°), as might
be expected for the reduced basicity of the Pt−L unit. Similarly,
⟨γ⟩ is larger (and ⟨θ⟩ is smaller) for the adduct in which the
chloro group is hydrogen bonding with water solvate (−0.35 Å,
147° vs −0.44 Å, 177°). The accumulated observations are
consistent with the notion that increasing the electron density
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on the metal center tends to enhance the NH···L/NH···Pt
interactions. It is apparent that these interactions are
comparable in strength to hydrogen bonds and that a
competing intramolecular hydrogen-bonding interaction can
effectively peel one of the piperidinium groups away from the
Pt−L unit, as in the case of the thiocyanate adduct. However, as
energy along the series NO2 > Cl > Br > SCN > I (Figure 4a).
This order is similar to that found for MLCT transitions of
2+
is found in structures of related Pt(diimine)(pipNHC)2
salts,29 in no instances are both piperidinium groups found to
interact with a basic solvate molecule or counteranion.
Electronic and Emission Spectroscopy. In order to
assess the influence of the dangling piperidyl and piperidinium
groups on the electronic structures of these complexes,
electronic absorption and emission spectra were recorded.
The yellow Pt(diimine)(pip2NCNH2)(L)+ salts dissolve in
CH2Cl2 to give yellow solutions. In keeping with the absorption
spectra of related platinum(II) complexes, an intense phen
ligand-localized (1LL) transition occurs near 275 nm.71,72 At
longer wavelengths, two distinct maxima occur in the 340−380
nm range, having spacings consistent with vibronic structure
(Table 5). A survey of the spectra of related complexes suggests
Figure 4. Electronic absorption spectra of (a) Pt(phen)(pip2NCNH2)-
(L)2+ in CH2Cl2, where L = Cl (light-green line), Br (red line), I
(purple line), NCS (orange line), and NO2 (dark-green line), and (b)
after the addition of 2 equiv of base in CH2Cl2, where L = Cl (light-
green line), Br (red line), I (purple line), NCS (orange line), and NO2
(dark-green line).
Table 5. UV−Visible Absorption Spectroscopy Data in
Ru(diimine)2L2 complexes in acetonitrile (NO2 > SCN > Cl >
Br > I).76 The slight difference in ordering is likely related to
the fact that the overall shift for the platinum(II) series is
substantially smaller (∼600 cm−1) than that for the ruthenium-
(II) series (∼3000 cm−1). For example, the shift in the
absorption band maximum upon substitution of NO2 for Cl in
Pt(diimine)(pip2NCNH2)(L)2+ (600 cm−1; Figure 4a) is less
than half that found for the longest-wavelength spin-allowed
MLCT band of Ru(tpy)(bpy)(L)+ (1400 cm−1).77,78 Both the
smaller shift and the broadening along the platinum(II) series
are consistent with stabilization of a MLCT state in the vicinity
of the LL state, resulting in band overlap and/or MLCT/LL
mixing. It is difficult to distinguish between these possibilities at
this time.
CH2Cl2
compound
wavelength (nm) (ε, 104 M−1 cm−1
)
[Pt(phen)(pip2NCNH2)Cl]
(PF6)2 (1)
278 (2.97), 305sh (1.07), 332sh (0.42),
348 (0.35), 366 (0.34)
Pt(phen)(pip2NCN)Cl
264 (2.85), 273 (2.95), 328sh (0.47),
378 (0.32)
[Pt(phen)(pip2NCNH2)Br]
(PF6)2 (2)
278 (3.23), 305sh (1.07), 333sh (0.42),
350 (0.36), 367 (0.36)
Pt(phen)(pip2NCN)Br
261(2.6), 276(2.83), 326sh (0.50),
378 (0.32)
[Pt(phen)(pip2NCNH2)I]
(PF6)2 (3)
279 (3.05), 300sh (1.16), 357sh (0.33),
370 (0.35)
Pt(phen)(pip2NCN)I
265 (3.2), 278sh (2.2), 363 (0.35),
438 (0.24), 500sh (0.10)
[Pt(phen)(pip2NCNH2)
(NCS)](PF6)2 (4)
279 (2.89), 305sh (1.04), 351 (0.36),
368 (0.38)
It is noteworthy that platinum(II) phenanthroline complexes
with phenyl donor ligands exhibit MLCT transitions in the
400−500 nm region79 [e.g., Pt(phen)(Ph)2, CH2Cl2, 437
nm]29 and that the longest-wavelength maximum for Pt(phen)-
Cl2 occurs at 396 nm, red-shifted by 2100 cm−1 from that of
Pt(phen)(pip2NCNH2)Cl2+. The remarkable destabilization of
MLCT states of Pt(phen)(pip2NCNH2)Cl2+ is attributable to
the interaction of one or both of the piperidinium NH groups
with the Pt−Cl unit, which tends to stabilize the metal-centered
orbitals. Accordingly, we note that the addition of 2 equiv of
TBAOH causes a significant red shift and broadening in the
long-wavelength region of the absorption spectrum (Figure
4b). Whereas the longest-wavelength absorption maximum for
the diprotonated phen adducts falls within a narrow 900 cm−1
range (358−370 nm), a comparably intense feature in the
spectra of the deprotonated complexes appears as a maximum
or shoulder over a 3600 cm−1 range (378−438 nm). This
sensitivity to the protonation state is remarkable given that the
piperidinium N atoms are four and five covalent bonds
removed from the metal center and phenanthrolinyl ligands,
respectively. The results are consistent with stabilization of a
transition having significant MLCT character, which is expected
to occur upon loss of the NH···Pt/L interactions. Similar results
have been observed for Pt(phen)(pipNHC)22+/Pt(phen)-
(pipNC)2 complexes, in which intramolecular NH···Pt
interactions destabilize MLCT states.29 It is intriguing that
the resulting 3600 cm−1 range of the 378−438 nm feature for
Pt(phen)(pip2NCN)(NCS)
268 (2.61), 297sh (1.06), 359 (0.37),
416 (0.26), 500sh (0.05)
[Pt(phen)(pip2NCNH2)
(NO2)](PF6)2 (5)
278 (2.97), 305 (0.92), 341 (0.28),
358 (0.23)
Pt(phen)(pip2NCN)(NO2)
266 (3.30), 280sh (1.90), 298sh (1.25),
405sh (0.23), 490sh (0.08)
[Pt(NO2phen)(pip2NCNH2)
Cl](PF6)2 (7)
282 (1.44), 314sh (0.56), 350sh (0.30),
367sh (0.21)
Pt(NO2phen)(pip2NCN)Cl
270 (1.57), 317sh (0.51), 400 (0.22)
that moderately intense metal-to-ligand charge-transfer
(MLCT) and LL transitions can occur in this region. For
example, Pt(phen)(pipNHC)2 gives rise to two comparably
2+
intense solvent-insensitive absorption maxima at 348 nm (3600
M−1 cm−1) and 365 nm (3800 M−1 cm−1).29 Although
variations in the electron-donor properties of ancillary ligands
(L′) of Pt(phen)(L′)2n+ can be expected to dramatically perturb
the energies of the charge-transfer (CT) states of platinum(II)
diimine complexes,73,74 two similar features appear in the 340−
380 nm range in the spectra of a wide array of phen derivatives,
regardless of the ancillary ligand.71,72,75 A notable example is
2+
the spectrum of Pt(phen)2 (352 and 373 nm), in which the
lowest spin-allowed MLCT band is clearly shifted to much
shorter wavelengths, yet there remain two distinct maxima near
352 and 373 nm,72 which we attribute to a LL transition. For
the Pt(phen)(pip2NCNH2)(L)2+ series, the two features
undergo a gradual broadening and a slight shift to lower
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the Pt(phen)(pip2NCN)(L) series exceeds the range observed
for Ru(diimine)2L2 complexes with two halide/pseudohalide
ligands (3000 cm−1).77,78 Moreover, the energy ordering of
these bands in the spectra of the platinum complexes is
somewhat different: Cl ≈ Br > NO2 > NCS > I. In the cases of
L = NO2, NCS, and I, there is an additional weak, but very
broad band near 500 nm (500−1000 M−1 cm−1). Spectral
changes accompanying the gradual addition of 2 equiv of base
to the chloro (phen and NO2phen), bromo, and iodo adducts
are consistent with an A → B → C process. For example,
titration of a CH2Cl2 solution of the bromo adduct results in
two sets of sharp isosbestic points (Figure 5). The behavior for
Table 6. Emission Spectroscopic Data in Butyronitrile
Solution
compound
conditions
emission maxima (nm)
466, 496, 531, 583sh
[Pt(phen)(pip2NCNH2)Cl]
(PF6)2 (1)
77 K, glassy
solution
[Pt(phen)(pip2NCNH2)I]
77 K, glassy
solution
469, 501, 534, 585sh
469, 499, 538, 584sh
519
(PF6)2 (3)
[Pt(phen)(pip2NCNH2)
(NCS)](PF6)2 (4)
77 K, glassy
solution
room
temperature
[Pt(phen)(pip2NCNH2)(NO2)] 77 K, glassy
(PF6)2 (5) solution
472, 497sh, 506, 543,
586, 632
diimine vibrational modes. The emission bandshapes are very
similar, confirming their relative insensitivity to the donor
properties of the monodentate ligands. In contrast, the
deprotonated complexes do not exhibit emission in the visible
region in a room-temperature solution, which is in keeping with
observations for other platinum(II) polypyridyl complexes with
dangling nucleophiles.26,27,29,80,85 This result is consistent with
stabilization of weakly emissive CT states upon deprotonation,
as well as the notion that interactions of the d8-electron metal
center with nucleophiles can quench luminescence.
Electrochemistry. To assess the influence of the dangling
piperidyl and piperidinium groups on the redox properties of
Pt(phen)(pip2NCNH2)Cl2+, Pt(phen)(pip2NCNH2)Br2+, and
Pt(phen)(pip2NCNH2)I2+, cyclic voltammograms (CVs) were
recorded of samples dissolved in CH2Cl2 with 0.1 M TBAPF6.
The CV of Pt(phen)(pip2NCNH2)Cl2+ (0.025 V/s, gold
working electrode) shows no evidence of oxidation at potentials
<1.5 V vs Ag/AgCl, and this is consistent with stabilization of
the amine groups by protonation. Similarly, no reduction
processes are observed at potentials >−0.5 V. We were
surprised to find that deprotonation of the piperidinium groups
did not effectively turn on the reversible two-electron chemistry
observed for Pt(tpy)(pip2NCN)+. The addition of 0.5 equiv of
TBAOH resulted in the appearance of an irreversible wave
characterized by an anodic peak (Epc), maximizing near 0.8 V
(Figure 6). The addition of 1 equiv of base caused a significant
increase in the current near 0.8 V, as well as the appearance of
an oxidation process at 0.5 V and an apparent back-reduction
process near 0.16 V. Further addition of base (up to 2 equiv in
total) caused a significant increase in the current associated
with the ∼0.5 V feature at the expense of the current associated
with the ∼0.8 V process. The peak-to-peak separation (ΔEp)
between the anodic (Epa, 0.5 V) and cathodic (Epc, 0.2 V) peaks
was >200 mV at all scan rates, indicating poor electrochemical
reversibility. The ratio of the cathodic to anodic peak currents
(ipa/ipc) was 2.75 at 0.01 V/s, suggesting that the electro-
chemical product is chemically unstable. Faster scan rates
improved the chemical reversibility, but even at scan rates up to
1.0 V/s, there was significant decomposition (ipa/ipc, 1.25).
Thus, under all conditions, it was apparent that this process is
less chemically and electrochemically reversible than that
observed for Pt(tpy)(pip2NCN)+ (e.g., ΔEp, 43 mV; ipa/ipc,
1.00 at 0.01 V/s).26,27 The CVs of both Pt(phen)(pip2NCN)Br
and Pt(phen)(pip2NCN)I exhibit similar oxidation processes
near 0.5 V. However, in the cathodic sweep, the behavior of
these adducts is more complicated, giving rise to multiple waves
(Figures S12−S14 in the Supporting Information). The exact
nature of the 0.8 V process is not certain. A similar wave
appears in the CV of Pt(phen)(pipNC)(pipNHC)+ (Epa, 0.8 V;
Figure 5. UV−visible absorption spectra recorded during the addition
of 100 μM TBAOH to a 50 μM solution of 1 in CH2Cl2. The addition
of (a) 1 equiv and (b) a second equiv in 0.1 molar increments.
the chloro, bromo, and iodo adducts is consistent with the
mono- and diprotonated adducts having different pKa values.
Interestingly, the addition of the first equiv of base has a
significantly greater impact on the longest-wavelength
absorption features than that of the second equiv. This effect
is dramatic in the case of the iodo adduct, in which the first
equiv of base shifts the longest-wavelength band by ∼4000
cm−1 to near 435 nm, whereas the second equiv only causes
broadening of the long-wavelength tail of that band (Figure S6
in the Supporting Information). Taken together with our earlier
report,29 these results demonstrate that electronic spectroscopy
is an exceptionally sensitive tool for probing intramolecular
NH···Pt and NH···L interactions.
Assignment of the long-wavelength bands in the deproto-
nated adducts cannot yet be made with certainty. We have
observed similar features in the spectra of a series of related
platinum(II) complexes with dangling nucleophiles and π-acidic
pyridyl ligands,26,27,80 including Pt(diimine)(pipNC)2 com-
plexes.29 It can be reasonably expected that the interaction of
dangling basic groups at open coordination sites of the metal
center will stabilize CT transitions having metal-to-pyridyl
ligand character. However, the extent of such interactions (if
any) in the Pt(phen)(pip2NCN)(L) series is not currently
known. Each diprotonated complex (L = Cl, Br, I, NO2, or
NCS) gives rise to intense green emissions in the solid state
and in a 77 K frozen solution. However, only Pt(phen)-
(pip2NCNH2)(NCS)2+ is emissive in a fluid solution, exhibiting
a weak and broad green emission near 519 nm. In 77 K dilute
butyronitrile frozen glassy solutions, each complex gives rise to
an intense blue-green emission, originating near 450 nm
(Figure S10 in the Supporting Information and Table 6). The
3
structured emission profiles are characteristic of a lowest LL
excited state.72,73,81−84 The vibronic structure with 1230−1630
cm−1 spacings is consistent with overlapping progressions of
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Figure 6. CVs of 1.0 μM 1 in 0.1 M TBAPF6/CH2Cl2 at 0.1 V/s: (a) 0.5 equiv of TBAOH; (b) 1.0 equiv of TBAOH; (c) 2.0 equiv of TBAOH.
0.01 V/s),29 and both pip2NCNBr and pip2NCNBrH22+ exhibit
an adsorption wave near 0.8 V. In contrast, Pt(pip2NCN)X (X
= Cl, Br, or I), for which the piperidyl groups are protected by
coordination with the metal center, is irreversibly oxidized near
1.15, 1.05, and 0.90 V vs Ag/AgCl, respectively; similar results
have been reported for Pt(Me4NCN)Cl (0.76 V vs Fc/Fc+, 0.1
M TBAPF6/THF).86,87
for a pyridyl group (i.e., Pt(phen)(pip2NCN)Cl vs Pt(tpy)-
(pip2NCN)+; Scheme 1). At least in part, the apparent
disagreement can be attributed to the non-Nernstian electro-
chemistry of Pt(phen)(pip2NCN)Cl, which cannot be expected
to provide a reliable estimate of E1/2. Nevertheless, this is
clearly not the entire explanation, and the influence of ligand
substituents on the relative stabilities of the platinum d6/d7/d8-
electron configurations remains an area of ongoing inves-
tigation.
The anodic wave near 0.5 V clearly arises from Pt(phen)-
(pip2NCN)(L). A similar wave was observed for Pt(phen)-
(pipNC)2 (Epa, 0.45 V) under identical conditions.29 By
analogy to Pt(tpy)(pip2NCN)+, we tentatively assign this to a
metal-centered process that involves the dangling piperidyl
groups. For Pt(phen)(pip2NCN)Cl, the values of the anodic
(Epa, ∼0.5 V) and cathodic (Epc, 0.2 V) peak potentials show a
characteristic dependence on the sweep rate, with Epa
undergoing a substantial anodic shift with increasing scan rate
compared to the cathodic shift of Epc. Epa increases from 0.46 to
0.60 V as the scan rate is varied from 0.025 to 1.0 V/s; over the
same scan rate range, Epc decreases by only 0.05 mV. This
pattern of behavior is very similar to that found for cooperative
two-electron reagents26,27,88−90 and is consistent with the
formation of an organized structure, such as a five- or six-
coordinate complex with intramolecular Pt−piperidyl inter-
actions, prior to or during anodic electron transfer. Such
interactions are likely to be weaker in the case of Pt(phen)-
(pip2NCN)Cl because of the lower electrophilicity of the metal
center compared to Pt(tpy)(pip2NCN)+. Thus, the compara-
tively lower platinum acidity of Pt(phen)(pip2NCN)L appears
to account for its reduced electrochemical reversibility.
The involvement of the piperidyl groups is reflected in the
cathodic shift relative to the PtIII/II couple of Pt(phen)-
(mesityl)2 (0.45 V vs Fc/Fc+; 0.1 M TBAPF6/THF),79 in which
there are no amine groups available to interact at the open
coordination sites. Given the strong donor properties of the
mesityl groups and the well-characterized relative influence of
the phenyl and chloro donor groups on RuIII/II couples,91 we
conclude that Pt(phen)(mesityl)2 provides a conservative lower
limit (i.e., ≥1.0 V vs Ag/AgCl) on the PtIII/II couple for
Pt(phen)(pip2NCN)Cl in the absence of apical nucleophile
interactions. Thus, the dangling amine groups stabilize the
oxidized complex by at least 0.5 V. For Pt(phen)(pip2NCN)Cl,
if the anodic (0.5 V vs Ag/AgCl) and cathodic (0.2 V Ag/
AgCl) peak potentials are taken as the upper and lower limits of
the formal redox potential (E1/2), then it would appear that E1/2
is surprisingly similar to the PtIV/II redox couple of Pt(tpy)-
(pip2NCN)+ (0.4 V vs Ag/AgCl; 0.1 M TBAPF6/CH3CN).26,27
For example, from the relative influence of chloride compared
to a pyridyl ligand on the RuIII/II couple of six-coordinate
complexes,92−95 we would anticipate a 0.4−0.5 V cathodic shift
in the Pt-centered couple upon substitution of a chloride group
CONCLUSIONS
■
The accumulated data indicate that the halide/pseudohalide
group (L−) and the metal center in Pt(phen)(pip2NCNH2)-
(L)2+ are capable of behaving as Brønsted bases, forming
intramolecular NH···Pt/L interactions involving the piperidi-
nium groups. In the case of the protonated adducts, the
geometries of the NH···Pt interactions are consistent with
hydrogen bonding to the metal, rather than three-center/two-
electron agostic interactions.26,68,96 The involvement of the
halide/pseudohalide group is reflected in the tendency of the
NH group to be directed toward the Pt−L bond, such that the
proton has the appearance of bridging the two bonded proton
acceptors. In conjunction with our earlier report,29 the CT
spectra of this series of complexes demonstrate that electronic
spectroscopy is a powerful and exceptionally sensitive method
for probing intramolecular NH···Pt and NH···L interactions.
Specifically, the NH···Pt/L interactions have a significant and
unambiguous impact on the electronic spectra of these
complexes, strongly destabilizing MLCT states and inducing
an anodic shift in metal-centered redox processes. By contrast,
deprotonation results in a new low-energy absorption band and
significant stabilization of higher oxidation states. These latter
properties are qualitatively similar to our observations for a
series of related systems with dangling nucleophiles.26,27,29,80
Interestingly, the chemical and electrochemical reversibility of
the Pt(phen)(pip2NCN)(L) adducts is substantially lower than
that of Pt(tpy)(pip2NCN)+. It is apparent that the mere
availability of dangling piperidyl groups that can stabilize a six-
coordinate d6-electron product, such as in Pt(phen)-
(pip2NCN)(L) and Pt(diimine)(pipNC)2, is not sufficient to
meet the requirements for reversible two-electron transfer. We
believe that a contributing factor is the lower electrophilicity of
the metal center in the Pt(phen)(pip2NCN)(L) series, which is
anticipated to decrease the tendency of the dangling piperidyl
groups to interact at the open coordination sites compared to
Pt(tpy)(pip2NCN)+. These interactions are expected to
preorganize the complex for electron transfer, thereby
increasing the rate of heterogeneous electron transfer and
conversion to the d6-electron product. In addition, it would
appear that the combination of a monodentate halide ligand
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̈
ASSOCIATED CONTENT
■
Trans. 1993, 2259−2260.
S
* Supporting Information
(17) Grant, G. J.; Spangler, N. J.; Setzer, W. N.; VanDerveer, D. G.;
Mehne, L. F. Inorg. Chim. Acta 1996, 246, 31−40.
Figures S1−S14 and crystallographic data in CIF format for all
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
(18) Blake, A. J.; Roberts, Y. V.; Schroder, M. J. Chem. Soc., Dalton
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Trans. 1996, 9, 1885−1895.
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AUTHOR INFORMATION
■
(20) Matsumoto, M.; Funahashi, S.; Takagi, H. D. Z. Naturforsch., B:
Chem. Sci. 1999, 54, 1138−1146.
Corresponding Author
*E-mail: bill.connick@uc.edu. Fax: (+1) 513-556-9239.
(21) Connelly, N. G.; Emslie, D. J. H.; Geiger, W. E.; Hayward, O.
D.; Linehan, E. B.; Orpen, A. G.; Quayle, M. J.; Rieger, P. H. J. Chem.
Soc., Dalton Trans. 2001, 670−683.
Notes
The authors declare no competing financial interest.
(22) Grant, G. J.; Carter, S. M.; LeBron, R. A.; Poullaos, I. M.;
VanDerveer, D. G. J. Organomet. Chem. 2001, 637−639, 683−690.
(23) Brown, K. N.; Geue, R. J.; Hambley, T. W.; Hockless, D. C. R.;
Rae, A. D.; Sargeson, A. M. Org. Biomol. Chem. 2003, 1, 1598−1608.
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Emslie, D. J. H. J. Am. Chem. Soc. 2003, 125, 8680−8688.
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2003, 125, 3446−3447.
ACKNOWLEDGMENTS
■
Funding for the SMART6000 CCD diffractometer was through
NSF-MRI Grant CHE-0215950. W.B.C. thanks the National
Science Foundation (Grant CHE-0134975) for their generous
support and the Arnold and Mabel Beckman Foundation for a
Young Investigator Award. S.C. thanks the University of
Cincinnati University Research Council for a summer fellow-
ship and the University of Cincinnati Industrial Affiliates
Program for their generous support. We also thank Dr. Allen G.
Oliver, Dr. Hershel Jude, and the late Professor Richard C.
Elder for helpful discussions and Drs. Larry Sallans and Stephen
Macha for assistance with MS measurements. Synchrotron data
were collected through the SCrALS (Service Crystallography at
Advanced Light Source) project at Beamline 11.3.1 at the
Advanced Light Source (ALS), Lawrence Berkeley National
Laboratory. The ALS is supported by the U.S. Department of
Energy, Office of Energy Sciences, under Contract DE-AC02-
05CH11231.
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